Squashed 'src/leveldb/' changes from ae6c262..a02ddf9

a02ddf9 Added GNU/kFreeBSD kernel name (TARGET_OS)
8487468 CondVar::SignalAll was broken, leading to deadlocks on Windows builds. http://code.google.com/p/leveldb/issues/detail?id=149
f6d84d1 Allow files to be opened for reading multiple times
cb8e3f7 Checking whether closing succeeds
d5317e8 Print actual Win32 error that occurred on file creation failure.
907f308 Port leveldb to MinGW32
9def2bf Mingw support for Windows LevelDB port
0a7b074 Pre-Vista leveldb::port::InitOnce implementation
31a2b09 Native Windows LevelDB port
058a035 Remove Snappy support
5bd76dc Release leveldb 1.12
7b094f1 Release leveldb 1.11
28dad91 Release leveldb 1.10
514c943 Make DB::Open fail if sst files are missing.
d84c825 Fix corruption bug found and analyzed by dhruba@gmail.com
ea2e919 added utility to dump leveldb files
REVERT: ae6c262 Merge branch 'leveldb' into ripple-fork
REVERT: 28fa222 Looks like a bit more delay is needed to smooth the latency.
REVERT: a18f3e6 Tidy up JobQueue, add ripple_core module
REVERT: ab82e57 Release leveldb 1.12
REVERT: 02c6259 Release leveldb 1.11
REVERT: 5bbb544 Rate limit compactions with a 25ms pause after each complete file.
REVERT: 8c29c47 LevelDB issue 178 fix: cannot resize a level 0 compaction set
REVERT: 18b245c Added GNU/kFreeBSD kernel name (TARGET_OS)
REVERT: 8be9d12 CondVar::SignalAll was broken, leading to deadlocks on Windows builds. http://code.google.com/p/leveldb/issues/detail?id=149
REVERT: c9fc070 Upgrade LevelDB to 1.10.0, mostly for better write stall logging.
REVERT: 8215b15 Tweak to variable name to support unity build
REVERT: aca1ffc Allow files to be opened for reading multiple times
REVERT: 693a70c Checking whether closing succeeds
REVERT: 0144d04 Print actual Win32 error that occurred on file creation failure.
REVERT: 43ed517 Fix corruption bug found and analyzed by dhruba@gmail.com
REVERT: 413c74c added utility to dump leveldb files
REVERT: 96eda85 Port leveldb to MinGW32
REVERT: 0967260 Mingw support for Windows LevelDB port
REVERT: ee3f9bd Pre-Vista leveldb::port::InitOnce implementation
REVERT: f5d0a41 Native Windows LevelDB port
REVERT: 28b35f1 Remove Snappy support

git-subtree-dir: src/leveldb
git-subtree-split: a02ddf9b14d145e88185ee209ab8b01d8826663a
Merge commit '84d6d69fc69662b2709fffbeaf3c3b4f53c535b1'
Merge pull request #2907 from sipa/leveldb-noripple

Switch to cleaned-up LevelDB 1.12 branch.